We have focused our recent efforts on studying the biosynthesis and targeting of newly synthesized lysosomal enzymes to lysosomes. The asparagine-linked oligosaccharide units of lysosomal enzymes undergo an extensive series of processing reactions as the newly synthesized enzymes move from the rough endoplasmic reticulum where they are initially glycosylated to their final destination in lysosomes. The enzymes which mediate these processing reactions are localized in a series of compartments within the cell, including the endoplasmic reticulum and several regions of the Golgi complex. The most important oligosaccharide modification is the generation of the phosphomannosyl recognition marker, which occurs in two steps. First, N-acetylglucosamine 1-phosphate is transferred to an acceptor mannose by UDP-N-acetylglucosamine:lysosomal enzyme N-acetylglucosamine 1-phosphotransferase, resulting in a phosphate group in diester linkage between outer N-acetylglucosamine and an inner mannose. Next, a phosphodiester glycosidase removes the N-acetylglucosamine, leaving the phosphate in monoester linkage to the underlying mannose residue. This exposed phosphomannosyl residue serves as an essential component of a recognition marker which leads to binding to high-affinity receptors and subsequent translocation to lysosomes. The N-acetylglucosaminylphosphotransferase acts specifically on lysosomal enzymes, thereby catalyzing the initial, determining step by which newly synthesized lysosomal enzymes are distinguished from other newly synthesized glycoproteins and marked for eventual targeting to lysosomes. We propose that all lysosomal enzymes contain a common protein sequence or conformation recognized by the N-acetylglucosaminylphosphotransferase. Our current experiments are directed toward the identification of this recognition signal. We have also identified a number of murine cell lines that lack the Man-6-P receptor and yet possess high levels of intracellular acid hydrolases which are contained in dense granules characteristic of lysosomes. These data indicate that there is a second pathway independent of the Man-6-P receptor for the delivery of lysosomal enzymes to lysosomes. We plan to try to elucidate the mechanism of this alternate pathway.